Conventional magnetic resonance imaging (MRI) uses powerful magnetic fields to align, that is, polarize the spin vector of protons, particularly protons inside the hydrogen nuclei in water molecules. Using RF excitation pulses, the system knocks the spin vectors out of alignment, and as they re-align, they produce a resonance signal that is used for imaging. This approach, however, only enables MRI scanners to polarize a small fraction of the water protons; for example, a 1.5 Tesla magnetic field, at room temperature, will polarize only about 3 protons out of one million. This inefficiency places limitations on the resolution, sensitivity, and dynamic contrast range of MRI. MRI finds extensive application, in part due to its sensitivity to the chemical characteristics of tissue components, in the characterization and differentiation of soft tissues. Other applications include fluid chemical analysis of small molecules and biomolecules (e.g. protein-ligand interactions, protein folding, protein structure validation, and protein structure determination), solid state analysis (structural), dynamics of time-variable systems, and the like.
Conventional MRI is characterized in that in sets up a highly uniform static main magnetic field (also called the B0 magnetic field), creating nuclear spin precession at a corresponding narrow band of resonance frequencies. A drawback is that the typical setup requires large magnets, gradient field coils, and radio frequency (RF) coils, adding to the bulk, complexity, and cost of the system.
Micro MRI systems exist that overcome some of these drawbacks. In one example, permanent magnets on the tip of a catheter generate a static magnetic field at the catheter tip. A micro MRI system also has a high quality receiving coil built into the tip, such as a Helmholtz micro coil. This allows for local imaging of blood vessels without the need for external magnets or coils. Gradient coils facilitate Fourier images or point-by-point imaging/analysis to be performed without gradient coils. Some advantages are low cost, patient accessibility, compatibility with existing tools, and high resolution. Drawbacks include the aforementioned problem of a small imaging region and only polarizing a few protons per million, but this is in part balanced by the proximity to the resonating protons.
Spin-exchange optical pumping techniques, using circularly polarized light, are able to increase noble gas dipole polarization to close to 100% that is, hyperpolarize these gases in a limited region. These methods, however, have only been shown to be suitable for hyperpolarizing low density noble gases, such as Xenon or Helium, under controlled laboratory conditions. Such techniques have been used for applications like contrast enhanced MRI of the pulmonary airways; a subject inhales the prepared gas (breathing air in which some of the nitrogen has been replaced with hyperpolarized Xenon), and then MR data are collected. Existing methods have not contemplated hyperpolarization of liquids or solids which would enable the standard MR imaging signal associated with blood and biological tissue to be enhanced.
The present application provides a new and improved optical polarization device which overcomes the above-referenced problems and others.